1. Field of the Invention
This invention relates to an internal pressure explosion-proof system, and more particularly to an internal pressure explosion-proof system for use with an electrically operated robot or the like which is used in an explosive atmosphere.
2. Description of the Prior Art
Various internal pressure explosion-proof systems of the type mentioned are already known, and an exemplary one of such conventional internal pressure explosion-proof systems is disclosed, for example, in Japanese Utility Model Laid-Open No. 63-131390. The internal pressure explosion-proof system is shown in FIG. 2. Referring to FIG. 2, the internal pressure explosion system is generally denoted by 1a and is constituted such that protective gas from a compressor 8 is fed by way of a supply pipe 4 and a pair of air regulators 15 interposed in the supply pipe 4 into an airtight vessel 3a and then into a plurality of airtight vessels 3b all installed in an explosive dangerous zone which is defined by an alternate long and short dash line a in FIG. 2. The protective gas is then exhausted outside by way of exhaust pipes 5 individually connected to the airtight vessels 3b. An electric appliance such as a servomotor M or the like which may produce a spark or fire is installed in the inside of each of the airtight vessels 3b. In order to prevent explosive gas from being admitted into the airtight vessels 3a and 3b, the internal pressure of each of the airtight vessels 3a and 3b is set to a level a little higher than the pressure of an explosive atmosphere therearound.
Further, in order to prevent explosive gas in an atmosphere around the airtight vessels 3a and 3b from being admitted into any of the airtight vessels 3a and 3b when the internal pressure of the airtight vessel 3a or 3b drops or when the airtight vessel 3a or 3b is disassembled for the inspection thereof, a pressure switch 10 for detecting an internal pressure of an airtight vessel 3b is provided for each of the airtight vessels 3b on the upstream side in the gas feeding direction as indicated by a widened arrow mark with respect to a master valve 17 which is disposed at a terminal end of the exhaust pipe 5 for the airtight vessel 3b. The pressure switch 10 functions also as pressure setting means for manually setting the internal pressure of the airtight vessel 3b or 3a to a predetermined level a little higher than the explosive atmosphere therearound.
Meanwhile, in case the motors M or the like have rested from operation for a long interval of time, the internal pressures of the airtight vessels 3a and 3b may have dropped to the same pressure as that of the explosive atmosphere therearound during such rest from operation and explosive gas in the explosive atmosphere may have been admitted into the airtight vessels 3a and 3b. Normally, when any of the servomotors M is to be started from such situation, the master valves 17 are opened so that gas in the pressure vessels 3a and 3b may be replaced by protective gas.
Another internal pressure explosion-proof system which is constructed to discharge gas existing at an innermost portion of an airtight vessel to the outside in advance before a servomotor is started after a rest from operation for a long interval of time is disclosed in Japanese Patent Laid-Open No. 61-125791. In this manner, an electrically operated robot such as, for example, a coating robot which is used in an explosive atmosphere normally includes an internal pressure explosion-proof system by which the safety of explosion-proof is assured with certainty.
However, the conventional internal pressure explosion-proof systems described above are disadvantageous in that, if a bend or choking of an exhaust pipe 5 takes place between the corresponding pressure switch 10 and master valve 17, ventilation in the pipe 5 will be insufficient. Consequently, it may be erroneously judged by the pressure switch 10 that the pressure within the airtight vessels 3a and 3b remains within a normal range while actually gas in some of the airtight vessels 3a and 3b has not yet been sufficiently scavenged by protective gas. Accordingly, the safety of explosion-proof cannot be assured sufficiently.